This invention relates generally to the transmission and storage of data and, more particularly, to managing response times in redundant arrays of inexpensive disks.
Digital video and television systems need high bandwidth data transmission and low latencies. Redundant arrays of inexpensive disks (RAID) support high bandwidth data transfers and very low latencies. RAID configurations employ redundancy and/or parity blocks to mask the failure of a disk.
RAID configurations divide a received data stream into a sequence of blocks and write consecutive blocks of the sequence to different disks in the array. To retrieve data, the RAID configuration reads the blocks from the disks of the array and reconstitutes the original data stream from the read blocks. To increase reception and transmission speeds, the RAID configuration may write to and read from the various disks of the array in parallel.
Individual disks of a RAID configuration will occasionally stall or respond slowly to an access request due to disk surface defects and bad block revectoring. During a slow response, the entire RAID configuration may wait while one disk transmits requested data. Thus, a single slowly responding disk can cause a long latency for a read operation from the RAID configuration.
For digital video and cable systems, one slowly responding disk can cause a disaster, because data needs to arrive at a video receiver at a substantially constant rate to keep the receiver's input buffer full. Continued long transmission latencies can deplete the input buffer. A receiver's input buffer is typically only large enough to store about 1 to 2 of video data, i.e. several megabytes of data. If a slow RAID configuration causes a transmission gap of longer than about 1 to 2, the receiver's input buffer may completely empty. If the receiver's input buffer empties, a viewer may perceive a noticeable pause in the video being viewed. Defect-free transmission of video requires that such pauses be absent.
RAID configurations are economically attractive, because they provide low latencies and high bandwidth data storage using inexpensive disks. But, contemporary inexpensive disks often have bad regions, which occasionally lead to bad block revectoring and slow disk responses. A bad region can cause a read, which normally lasts about 10 milliseconds (ms), to take 1,000 ms or more. Thus, slow responses can cause unpredictable read latencies. These latencies make RAID configurations less acceptable in video transmitters, because transmission latencies can lead to the above-discussed problems in video reception.
The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.